Diagnostic utility of LDH measurement for determining the etiology of modified transudate pleural effusion in cats

Abstract

Fluid analysis is an initial approach for determining the underlying causes of body cavity effusions. Modified transudate is commonly diagnosed in pleural effusion in cats, however, it provides limited diagnostic information. Aims of this study were to investigate common etiologies causing different pleural fluid types and to evaluate the usefulness of lactate dehydrogenase (LDH) for differentiating the etiology in modified transudates in cats. Pleural effusion samples from 122 cats were analyzed and classified into three types: transudate, modified transudate, and exudate. Causes of pleural effusion were classified into four conditions: cardiac disease, neoplasia, feline infectious peritonitis (FIP), and pyothorax. The relationship of underlying etiology and fluid types was described. The LDH levels in pleural fluid and plasma were compared between the causes in the samples classified as modified transudate. The fluid analysis of pleural effusion showed that modified transudate was the most common fluid type (44.2%). Neoplasia was predominantly diagnosed (38.5%) as the etiology of pleural effusion. There was no significant correlation between pleural fluid and plasma LDH level in any type of pleural fluid, suggesting that pleural fluid LDH does not appear to be affected by plasma LDH. The occurrence of modified transudate was not associated to its etiologies, however, the LDH level in modified transudates showed significant differences between etiologic groups. The LDH level in modified transudate was excellent in separating cardiac from non-cardiac diseases with a cut-off value of <535 U/L and separating FIP from non-FIP diseases with a cut-off value of >641 U/L. Based on the current findings, pleural fluid LDH can be a useful adjunctive marker for differentiating some causes of modified transudate pleural effusion and should be added in the routine diagnostic work-up of feline patients with pleural effusions.

Keywords: cat; etiology; lactate dehydrogenase; modified transudate; pleural effusion.

Figure 1

Cytology of pleural effusions. (A) Pyogranulomatous effusion from pyothorax showing intracytoplasmic cocci bacteria in vacuolated macrophages and degenerate neutrophils. (B) FIP effusion showing non-degenerate neutrophils, macrophages, and few small lymphocytes. (C) Monomorphic population of large lymphoblasts suggestive of lymphoma. (D) Clusters of epithelial cells showing criteria of malignancy including anisocytosis, pleomorphism, and atypical mitosis suggestive of malignant epithelial tumor. Giemsa staining. Scale bars = 20 μm (A); 30 μm (B–D).

Figure 2

Scatter plots demonstrating the correlation of parameters in pleural fluid and plasma. (A) No significant correlation between pleural fluid and plasma LDH levels (Overall: r = 0.298; P = 0.05) in transudates (r = 0.451; P = 0.446), modified transudates (r = 0.203; P = 0.391), and exudates (r = 0.269; P = 0.252). (B) Significant correlation between pleural fluid and plasma TP concentration (Overall: r = 0.394; P < 0.01) in exudates (r = 0.176; P = 0.391), modified transudates (r = 0.585; P < 0.01), and transudates (r = −0.129; P = 0.808).

Figure 3

Box plots of LDH levels in pleural fluid (A) and plasma (B) of cats with modified transudates showing significant differences of LDH activity between groups in pleural fluid (P < 0.001) but not in plasma (P = 0.766). FIP, feline infectious peritonitis. The boxes represent the 25^th and 75^th quartiles with a horizontal line at the median. The whiskers represent the range of the data. Open dots represent the outliners. Numbers of analyzed specimens are indicated.